The 3GPP Release 12 specifications define Dual Connectivity (DC) (see, for example, Non Patent Literature 1 and 2). DC is similar to but different from Coordinated Multipoint (CoMP) introduced in the 3GPP Release 11 specifications. In particular, in DC, different carrier frequencies are used for a macro cell and a small cell (picocell), a radio terminal (i.e., User Equipment (UE)) executes two Medium Access Control (MAC) entities (that is, the UE uses radio resources provided by two independent schedulers, one of which is in a master base station (i.e., Master eNodeB (MeNB)) and the other is in a secondary base station (i.e., Secondary eNB (SeNB))), and the UE needs to support simultaneous use of at least two UL carriers.
In DC of 3GPP Release 12, the UE is connected to one MeNB and one SeNB. The UE configured with DC (DC UE) uses two MAC entities and at least two Radio Link Control (RLC) entities for respective data flows on the MeNB and the SeNB. The MeNB in DC is an eNB that terminates S1-MME for the DC UE. The SeNB in DC provides additional radio resources for the DC UE. One or more serving cells provided for the UE by the MeNB are referred to as a Master Cell Group (MCG), and one or more serving cells provided for the UE by the SeNB are referred to as a Secondary Cell Group (SCG). The MCG is one or more serving cells associated with the MeNB and includes a Primary Cell (PCell) and optionally one or more Secondary Cells (SCells). The SCG is composed of one or more serving cells associated with the SeNB and includes a Primary Secondary Cell (PSCell) and optionally one or more Secondary Cells (SCells). In DC of 3GPP Release 12, the PSCell is a special cell in the SCG and is different from a normal SCell in that the PSCell is configured with the uplink (i.e., PUCCH) for the UE.
In the control plane architecture for DC, there is only one S1-MME per DC UE. The S1-MME is defined between the MeNB and a Mobility Management Entity (MME), and the MeNB terminates the S1-MME. The signaling between the MeNB and the SeNB regarding the DC UE is performed using a signaling interface (i.e., X2-C) between the eNBs.
For DC, two user plane architectures are allowed. In the one architecture, S1-U is only terminated in the MeNB and the MeNB forwards user plane data to the SeNB using X2-U. In the other architecture, S1-U can also be terminated in the SeNB.
Accordingly, three types of radio bearers, i.e., MCG bearer, split bearer, and SCG bearer, are defined for DC. Regarding the MCG bearer, the MeNB terminates an S1-U connection with a Serving Gateway (S-GW), and Radio protocols of the MCG bearer are only located in the MeNB to use MeNB resources. Thus, the SeNB is not involved in transferring user plane data of the MCG bearer on the Uu interface.
Regarding the split bearer, the MeNB terminates an S1-U connection with an S-GW, and Packet Data Convergence Protocol (PDCP) data (i.e., PDCP Protocol Data Unit (PDCP PDU)) is transferred between the MeNB and the SeNB. Thus, both the MeNB and the SeNB are involved in transferring user plane data of the split bearer on the Uu interface.
Regarding the SeNB bearer, the SeNB terminates an S1-U connection with an S-GW, and Radio protocols of the SeNB bearer are only located in the SeNB to use SeNB resources. Thus, the MeNB is not involved in transferring user plane data of the SCG bearer on the Uu interface.
Some overall procedures regarding DC are defined in Non Patent Literature 1 (see, for example, Section 10.1.2.8 of Non Patent Literature 1).
SeNB Addition Procedure:
This procedure is initiated by the MeNB to establish a UE context at the SeNB. This procedure is used to add at least the first cell (i.e., PSCell) of the SCG. In this procedure, there is always at least one bearer (split bearer or SCG bearer) configured in the SCG.
SeNB Modification Procedure:
This procedure is used to modify the SCG in the SeNB and is initiated by the MeNB or the SeNB. This procedure is used to modify, establish or release bearer contexts, to transfer bearer contexts to and from the SeNB, or to modify other properties of the UE context within the same SeNB. The MeNB uses this procedure to add or release SCG SCells, SCG bearer(s), and the SCG part of split bearer(s). The MeNB also uses this procedure to trigger PSCell change involving PSCell release.
SeNB Release Procedure:
This procedure is initiated by the MeNB or the SeNB and is used to initiate the release of the UE context at the SeNB.
Intra-MeNB Change Procedure:
This procedure is used to perform handover within the same MeNB while keeping the SCG in the same SeNB.
SeNB Change Procedure:
This procedure is used to change the SCG between SeNBs (inter-SeNB). This procedure is initiated by the MeNB and used to transfer a UE context from a source SeNB to a target SeNB and to change the SCG configuration in the UE from one SeNB to another SeNB. This procedure is basically implemented by a combination of the SeNB Addition procedure and the SeNB Release procedure.
MeNB to eNB Change Procedure:
This procedure is initiated by the MeNB. This procedure is used to transfer context data from a source MeNB and a source SeNB to a target eNB.
Signaling and transferring of user plane data between the MeNB and the SeNB in DC are performed through the X2 interface. Accordingly, the following X2 procedures for DC are defined (see, for example, sections 8.6 and 9.1.3 of Non Patent Literature 2). Some of the above-described overall procedures use these X2 procedures for DC.                SeNB Addition Preparation procedure        MeNB initiated SeNB Modification Preparation procedure        SeNB initiated SeNB Modification procedure        MeNB initiated SeNB Release procedure        SeNB initiated SeNB Release procedure        
In 3GPP Release 12, when a UE is handed over from the MeNB to another eNB (target eNB), the above-described MeNB to eNB Change procedure is used. In this procedure, the SeNB (SCG) is completely released. Therefore, when the SCG is needed after the handover, the target eNB should perform the SeNB Addition procedure again after the completion of the handover. That is, the MeNB to eNB Change procedure requires, in addition to a normal handover procedure, signaling similar to the SeNB Change procedure (i.e., a combination of the SeNB Release procedure and the SeNB Addition procedure). Further, regarding the SCG bearer, data forwarding from the SeNB to the source MeNB and data forwarding from the source MeNB to the target eNB need to be performed.
In 3GPP Release 13, support of an Inter-MeNB handover from the source MeNB to the target MeNB without SeNB change, or support of an Inter-MeNB handover involving the SeNB Addition procedure, is being discussed. When these procedures are supported, the target eNB may not need to perform the SeNB Addition procedure to configure the SCG again at the SeNB. In addition, data forwarding regarding the SCG bearer from the SeNB to the source MeNB and from the source MeNB to the target eNB may become unnecessary.
Patent Literature 1 also discloses some procedures for a handover of a radio terminal from a first master base station to a second master base station during Dual Connectivity. In one of the procedures disclosed in Patent Literature 1, when a radio terminal in Dual Connectivity is handed over from the first master base station to the second master base station, the first master base station requests a secondary base station to suspend providing services in a SCell, and then the secondary base station suspends providing the services in the SCell and keeps communication status information about the SCell. Further, in this procedure, the second master base station sends an instruction to resume communication to the secondary base station after the handover and, in response to the instruction, the secondary base station resumes the services in the SCell based on the communication status information about the SCell, which is kept in the secondary base station. The communication status information about the SCell includes, for example, at least one of (a) transmission or reception status of user data (User Plane (U-plane)), (b) service information, (c) bearer information, and (d) radio resource configuration information.